Improved heat exchanger performance is extremely important in air-conditioning and refrigeration systems,
since it can lead directly to volume, weight, energy, and cost savings. The air-side heat transfer coefficient is the
limiting factor for many of these heat exchangers, and improvements in this heat transfer performance can yield
superior heat exchangers. This research investigates a novel approach for active air-side heat transfer enhancement
in offset-strip fin arrays through the use of thin, mechanically oscillating vanes placed upstream of a model offset-strip
fin array. Dye-in-water flow visualization, hot-wire anemometry, thin-film heat transfer characterization, and
static pressure drop experiments are performed to determine the thermal behavior of this actively enhanced heat
exchanger, over a range of forcing frequency/amplitude combinations for Reynolds numbers (based on hydraulic
diameter) from 450 to 1400.
Flow visualization experiments reveal dramatic changes in the flow through the array for Reynolds
numbers greater than 650 when the mechanical forcing is tuned to frequencies at or below the natural array vortex
shedding frequency with amplitudes equal to half the thickness of the fins within the array. These flow structures are
qualitatively similar to those associated with an increase in array-averaged heat transfer, as determined from
previous heat transfer experiments. It is also observed that forcing above the natural array vortex-shedding
frequency causes the flow structures within the array to be suppressed, returning the flow to that of the unforced
case.
Heat transfer characterization experiments reveal array-averaged enhancements up to 7.6% (+/- 4.5%) for
Re = 1400 when forced at the fin-shedding frequency with an amplitude equal to half the fin thickness. This increase
in heat transfer is accompanied by an increase in pressure drop of about 3% (+/- 4%), making the realized pressure
drop penalties are of the order of the experimental uncertainty. Row-by-row heat transfer experiments revealed fin averaged
heat transfer enhancements up to 14%. The largest enhancements in fin-averaged heat transfer are realized
for the center rows of fins of the array when forced appropriately for Re > 1200.

Issue Date:

2005-06

Publisher:

Air Conditioning and Refrigeration Center. College of Engineering. University of Illinois at Urbana-Champaign.